As the demand for hydrocarbon-based fuel has increased, the need for improved processes for separating hydrocarbon feedstocks of heavier molecular weight and higher contaminant levels has increased as well as the need for increasing the conversion of the heavy portions of these feedstocks into more valuable, lighter fuel products. These heavier, “challenged” feedstocks include, but are not limited to, low API gravity, high viscosity crudes from such areas of the world as the Middle East, Mexico, Venezuela, Russia, as well as less conventional refinery feedstocks derived from such sources as bitumen, shale oil and tar sands. It is also important that heavy crude fractions, such as atmospheric resids, vacuum resids, and other similar intermediate feedstreams containing boiling point materials above about 850° F. are processed in such a manner so as to improve their ability to be utilized as feedstreams for refinery catalytic conversion processes. These catalytic conversion processes are vital economic components of a modern refinery system as they are utilized to improve the overall conversion of the feedstream into commercially valuable fuel and specialty petrochemical products.
A major problem that exists with these high molecular weight feedstreams is that in their raw state, these streams often possess relatively high Conradson Carbon Residue (“CCR”) values which can range from about 1 to about 30 wt %. The CCR value of a hydrocarbon stream is an indication of the amount of carbon in a unit amount of the stream. Hydrocarbon streams that contain high CCR values, especially in excess of about 2 to 5 wt %, can be undesirable for use in some refinery catalytic conversion processes as they tend to increase the amount of coke in the refinery catalytic conversion processes and deactivate the catalysts at an unacceptable high rate for practical or optimal commercial use of these high CCR containing feedstreams to the conversion processes. As a result, these feedstreams are often downgraded to a lower value process, or mixed with other, lower boiling point hydrocarbon streams prior to processing in the conversion units. This latter processing scheme results in a decrease in the amount of overall heavy hydrocarbon feed that a given conversion unit can process and can still result in above optimum catalyst coking and catalyst deactivation rates.
In U.S. Pat. No. 4,814,088 to Kutowy et al., a polysulfone membrane was utilized to improve several heavy oil feeds. The Examples show the membrane to be effective in removing metals and reducing the viscosity. However, the process requires a sulfone based polymer membrane which further requires an initial swelling step and is limited to use in low viscosity feedstreams (below 600 centipose), requiring either the feed content to restricted in composition, or the system to be maintained at temperatures high enough to maintain the feedstream at this low viscosity. The use of a diluent is also suggested to maintain the viscosity within functional limits.
Similarly, U.S. Pat. No. 4,797,200 to Osterhuber et al. utilizes a cellulose or polyvinylidine fluoride polymer membrane in conjunction with a diluent to separate remove metals and reduce the microcarbon residue of a heavy hydrocarbon feed. The disclosed process is limited to pressures of about 215 psig (1500 kPa) and temperatures of about 257° F. (125° C.).
Other membrane materials, such as ceramics, have been utilized in the past, but have experienced certain limitations. In U.S. Pat. No. 5,785,860 to Smith, a ceramic membrane was utilized to separate a heavy crude oil stream. However, this process requires the permeate to be recycled to the feedstream for a period of time to condition the membrane prior to use. As a result, the pore structure of the membrane is to fouled reducing pore size and thus impacting the flow properties of the membrane.
U.S. Pat. No. 5,173,172 to Adams et al. utilizes a membrane separation process to make an 85/100 penetration asphalt. This process utilizes polymeric or ceramic membranes with process pressure and temperature limitations of 30 to 400 psig and 302 to 392° F. (150 to 200° C.), respectively.
Therefore, there exists in the industry a need for improved low energy membrane separations processes for economic reduction of the CCR content of a heavy oil feed.